[llvm] [LAA][NFC] Refactor deref no-wrap check; expose broken reverse-loop bounds (PR #211960)

[Aleksandr Popov via llvm-commits]

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@@ -277,45 +281,51 @@ static bool evaluatePtrAddRecAtMaxBTCWillNotWrap(
     LoopGuards.emplace(ScalarEvolution::LoopGuards::collect(AR->getLoop(), SE));
   MaxBTC = SE.applyLoopGuards(MaxBTC, *LoopGuards);
 
-  const SCEV *OffsetAtLastIter =
-      mulSCEVNoOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
-  if (!OffsetAtLastIter) {
+  const SCEV *AbsStep = SE.getAbsExpr(Step, /*IsNSW=*/false);
+  // Total distance (in bytes) walked between the first and the last
+  // accessed pointer; MaxBTC * |Step|.
+  const SCEV *WalkBytes = mulSCEVNoOverflow(MaxBTC, AbsStep, SE);
+  if (!WalkBytes) {
     // Re-try with constant max backedge-taken count if using the symbolic one
     // failed.
     MaxBTC = SE.getConstantMaxBackedgeTakenCount(AR->getLoop());
     if (isa<SCEVCouldNotCompute>(MaxBTC))
       return false;
-    MaxBTC = SE.getNoopOrZeroExtend(
-        MaxBTC, WiderTy);
-    OffsetAtLastIter =
-        mulSCEVNoOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
-    if (!OffsetAtLastIter)
+    MaxBTC = SE.getNoopOrZeroExtend(MaxBTC, WiderTy);
+    WalkBytes = mulSCEVNoOverflow(MaxBTC, AbsStep, SE);
+    if (!WalkBytes)
       return false;
   }
 
-  const SCEV *OffsetEndBytes = addSCEVNoOverflow(
-      OffsetAtLastIter, SE.getNoopOrZeroExtend(EltSize, WiderTy), SE);
-  if (!OffsetEndBytes)
+  // Total length in bytes of the accessed range (from the first accessed
+  // byte through the end of the last access); WalkBytes + EltSize.
+  const SCEV *SpanBytes = addSCEVNoOverflow(
+      WalkBytes, SE.getNoopOrZeroExtend(EltSize, WiderTy), SE);
+  if (!SpanBytes)
     return false;
 
+  // Compute MaxOffset per direction: exclusive upper offset of the
+  // accessed range.
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aleks-tmb wrote:

I initially introduced some of these changes; please see the original commit: https://github.com/llvm/llvm-project/pull/211960/changes/172ef03d3d19e2d7ceca01079e59b3e6834d57e9

However, during the review, we decided to preserve the existing terminology and move those changes to a follow-up patch containing the actual fix: https://github.com/llvm/llvm-project/pull/211960#discussion_r3674208932


https://github.com/llvm/llvm-project/pull/211960